Upvote:2
The general rule is that one must apply for Schengen visas in one's county of residence. Being in the UK on a tourist trip doesn't make your acquaintance a resident there, so ordinarily he wouldn't be allowed to file a visa application there.
The consulates are allowed to consider visa application from non-resident, if he can provide a good and convincing reason why he couldn't apply from his home country. Simply wanting to go on a pleasure trip won't cut it; a suddenly arising emergency or business need might well do.