The UK has a visa waiver for transit passengers (they're not that idiotic, they want Heathrow to keep being the big hub that it is; you do get to talk to an officer, but you generally don't need to apply in advance). Generally UK immigration is bad (like having to turn up and give your fingerprints every time you apply for a visa, so no application by post; in my case, that means a flight of 1,100 km and back every 6 months). Here's a blog post according to which being on a dissertation defense committee also requires a work permit (according to possibly overzealous HR): http://timesonline.typepad.com/dons_life/2010/10/world-class...

Most Schengen countries are more or less straightforward per se, though I'm not sure how his case would pan out (here in Norway, according to the immigration authority's webpage, "researchers and lecturers" are exempt from the work permit requirement, though I'm not sure how broad that category is).