Hiya, not much in the way of advice really, but just wanted to say I hope you find a workable solution.
It might be worthwhile your friend keeping Ollie in for a week or so before he comes to stay with you, that way he'll be used to being indoors again for a bit.
I would be a little concerned about Charlie not being vaccinated though, as you never know what Ollie may be carrying. My boy is indoors, and I don't vaccinate him, but that 'cos he has FIV. If Charlie is healthy, I would recommend you vaccinate him as a matter of course, as airborne viruses still exist as well.
Also, do you flea Charlie? If not, I would defo recommend you get him on a course of treatments (from the vet as over the counter ones are rubbish, and sometimes dangerous), as Ollie may carry in a flea and that's the last thing you want, especially over Christmas
Sorry I can't be of more help, just a few of my thoughts
x
Modified to add: it will also be worthwhile checking whether Ollie is still using a litter tray as he's going outside now.