Are you protected in an untrusted embassy in a country you trust?

In short, your friend probably has no choice but to enter the embassy. Unless it’s possible to give another person (e.g. a solicitor) mandate in writing to apply for renewal of the passport in your name.

(To answer the actual question: No, this is not “safe” if your friend does not trust his own country. The embassy is legally not part of the UK, so technically you are in your home country, under your country’s laws and under control of your country’s executive. Without a really, really, really urgent reason if you are a really, really important person, no UK institution will interfere with you being arrested in there.)

You stated that your friend has entered the UK with a valid visa. This means two things: First, he is rather obviously not a refugee but a visitor. Second, he is staying illegally if the passport expires during the stay.

Note:

You must have a valid passport to enter the UK. It must be valid for the whole of your stay. You may also need a visa, depending […]

So, you basically have the choice of risking a visit to the embassy to get the passport renewed, or you’re risking to be put on the list of illegal aliens along with the biometric stuff that you had to provide when applying for the visa (in the one country with the most surveillance cameras in the world).

If they catch you, and they likely will, chances are that you are expelled, and I wouldn’t want to wager whether shouting out “Asylum!” at that point will prevent this from happening when you’re an obvious illegal. In any case, being expelled will raise attention whereas having your passport renewed might, with some luck, just happen silently.

Talking to a lawyer may be an excellent idea.

If your friend is a Syrian citizen, then I have a thing or two to say regarding this since I live in a country with more than 2 million Syrian refugees, they’re not called refugees here, they have special treatment allowing them to move freely, work and study.

However, having many Syrian colleagues who have been through the same exact issue as your friend, I can assure you it will be fine. They might be called names in the embassy, but that’s it. Syrian government can’t afford to make things worse and lose the little support they have by doing something stupid as this.

Unless your friend is a high ranking member of one of the militias, or a close relative to one, then nothing should happen to him/her. Even if he was, I suspect he will be residing with the knowledge of the host country, and he/she will be knowing what to do.

In fact, it’s known that the embassy will allow them to get a passport, hoping to have them travel to Syria so they can do the “magic” there, not in a foreign country especially a place like the UK where they try hard to clear their reputation of being mass murderers and so.

I also know a colleague who did not go to Syria since the beginning of the revolution, she renews her passport with no issues, yet she knows if she goes there she might not come back (according to her), she also claims that members of her family have disappeared (in Syria) for being a part of the revolution, but again they are just common people not high ranking members.

No. If you do not trust them, do not go there.

Not in “that part of town” where “everybody knows everybody” including foreign staff of the local embassy, local police, local court personnel, local judges, local magistrates, local lawyers, local attorneys, local barristers, … All politics is local, they say.

You do not want to be “picked up” on a trumped-up miscellaneous trespass or other misdemeanor in “that part of town” and deported.

Establish a valid mailing address, and contact them preferably by mail, possibly by telephone, order your documents by mail, be very polite, and do not let them on to your suspicion.

Assuming that your friend is in a conflict with the bureaucracy of his home country, a much more likely problem is that the consulate will simply refuse to extend the passport and your friend would be forced to either leave the UK or apply for asylum there. Kidnapping, assaulting or killing someone at a foreign consulate is guaranteed to cause a diplomatic scandal and it’s unlikely that the local diplomatic staff will resort to that option.

Otherwise, the police generally would not be able to assist your friend right away. That’s the whole point of the Vienna Convention. If the UK wasn’t willing to extract Julian Assange out of the Ecuadorian embassy (granted that he’s staying there willingly), they’re unlikely to be willing to help out your friend.

An embassy’s premises are “inviolable” by the authorities of the host country, so the host country couldn’t prevent the embassy from detaining anyone there. The embassy would have a hard time getting someone out of the host country without the cooperation of host country authorities, however.

Of course, even if the UK could protect your friend, there is no guarantee that they would. They might not know about his plight, or they might not care about it. And even if they did want to help him in such a situation, he could nonetheless end up spending the rest of his life in that embassy.

If your friend truly fears for his safety, he should talk to a good immigration lawyer about whether it would be a good idea to apply for asylum. If such an application were successful, your friend would no longer need a passport from, or any other contact with, his country of citizenship.