OMG! I'm cracking up!!!! You have no idea how ready I was to ask what type of crack pot you were working with and then ask why you just don't 'get yourself a fish tank with a lid and dump the kitty into it!'
🤣
OK, now, how 'young' are we talking and how 'senior' are we talking? Also, have either ever lived with other cats prior to this? Small apartments can be really hard for intros; but if the kitties are willing, then it should go smoothly...
The first thing they should do is keep the 'new' kitty in his/her carrier until the resident cat comes to have a sniff - if there's some hissing, they shouldn't be surprised; but if not, they'll have a better time going about things. Also, if they can keep the 'senior' in a bedroom during the day and let the young one have roam of the apartment during the day, it may be helpful.
Two litter boxes are a MUST at the start as are separate eating areas. However, depending upon how small of an apartment your friend has, the distance may not be enough to 'please' the felines.
If all else fails, I like CT's idea about the 'playpen' - those are always a great way to introduce kitties!!
Sorry I can't be of more help - without knowing the personalities of the cats in question, or the layout of the space, it's kind of tough to give good advice.
Best of luck to your friend and the kitties!!!!
