Upvote:3
As stated on the UK immigration website:
You wonβt be able to transit without a visa if a Border Force officer decides you donβt qualify under the immigration rules. You can apply for a transit visa before you travel if youβre unsure whether you qualify for transiting without a visa.
So if you want to be 100% sure you'll pass without issues, apply for an entry clearance. Otherwise we cannot predict the outcome of your particular trip.
Upvote:3
The transit areas of UK airports close overnight, so if the connection is overnight, the passenger must enter the UK and as such is on much stronger ground.
They should present their passport at the page with the US visa, the boarding passes and the filled-out landing card.
If presenting these documents, and provided the connecting flight leaves no later than 23:59 the next day, you've fulfilled the requirements in terms of documentation.
Not qualifying under the immigration rules means either not having the necessary documents for the transit or not intending to actually transit. If the officer suspects the latter, i.e. that you'll abscond once in the UK, they could ask for additional evidence of your plans to leave the UK, such as ties to Pakistan.
However, from your description of the person, it is unlikely they'll arouse any major suspicion, particularly with a connection of less than 24 hours. If they're extremely worried, they could bring a hotel booking confirmation and a certified letter from their employer/school in Pakistan.
Most importantly, always answer any questions truthfully, and never have the mindset that certain questions are "private" or "intrusive". They ask something, you need to answer clearly, truthfully, and without hesitation or nervousness.
If you want "feedback" from the Border Force, you can call them at Heathrow terminal 3 (+44 20 8745 6941) or Heathrow terminal 5 (+44 20 8196 2500). You should, however, ask clear and concise questions to avoid confusion and misunderstandings.