Not a ULIP student but I have been studying in Paris for the latter half of last year as part of a Year Abroad so have a little bit of insight. Much like @Boosterquery 's DS, I met quite a few ULIP students while I was there!
I rented alone and got along very happily, although I had the advantage that I already knew other people from my uni etc. who were also there. Similarly, cohorts of ULIP students arrive every year so there will be other English students that he will know before going (it may be that he would live with them) and there are tons of English students in Paris, so the social scene is good! If he's already an outgoing lad, it may not be that different from moving away from home to a UK university with the obvious added pressure of being abroad.
Like you mentioned, costs are extortionate - a lot of people I knew were paying somewhere between $800-$1000 a month for their flats/houseshares. My flat was less than that, but very small - as with many things, the further in advance you sort it, the cheaper prices will be. General cost of living is also significantly higher, though not dissimilar to living in London, (without factoring in travel costs). I and other students managed with student loans and Erasmus grants. The Erasmus grant, to the best of my knowledge, was not available to ULIP students doing their full degree in Paris, but now it is the Turing Scheme it may be completely different, I have no idea.
The main drawback I would highlight is that if his main goal is to improve his French, ULIP is not the place to go IMO. ULIP students I met said almost without fail that they spoke very little French, as their classes were all in English, all of their friends are English, and everyone in Paris speaks English! As @TizerorFizz says he is probably better off studying over here but looking to do a Year Abroad. He would still have the chance to go to Paris if he wanted but would be able to study/work in French, or even head to somewhere else in France where English is less ubiquitous!
All that being said, I had an amazing time in Paris, it really is an incredible city and I'm sure if he did go, he'd enjoy it. However, as you mentioned he's not particularly wedded to the idea of ULIP, it's probably worth looking at some other options.
Apologies for the hugely long post but hope some of the info is useful! Feel free to PM me if you want to know any more specifics about living in Paris etc.