Am i the only one that think there could be a sinister purpose to that story? The whole impregnating herself kinda feel ridiculous to me... But keeping the cum as some form of blackmail seem more possible. Im not saying its what happened (and oh boy i may scare the OP further) but basically when a woman accuse a man of rape, what can be used as proof? Of course a rape kit at the hospital is likely best answer but we all know not everyone goes for it (must be done fast). But residue of cum on panties or such is considered a proof that can be received and this ... well it last a while.
Now sure these days even "her words against his words" can lead to convictions, we seen it happen, for allegled rapes that happened decades ago. But here having some old cum could be a good proof something happened. Guy would need to prove harder it was consensual (and in this context she is an escort) and even if he does, it could damage his life.
Honestly the whole idea of keeping the condom may not be a "bad idea" when you look at it. Sure if you see only very safe providers with great reputation its very unlikely you will have anything to worry about, but some random encounters out of LL? Who knows... Of course there is many other "DNA left" at an incall if someone really want to screw with you (hairs in shower/bed comes to mind) but i just feel sperm leftovers is "easier" to use.
He didn't mention who it was, so no idea if she is a well known providers, but i find it odd that she was asking all these questions about him going back to the bathroom or such. I mean its only been a shortwhile i switched to incall as my primary way of meeting, but i never had an SP inquire why i would return to the bathroom or anything (in my case i like to do a last quick check to make sure i didn't forgot some of my shower stuff or even a pair of boxer) . All SPs i seen were very chill and didn't followed me anywhere.
As a last note on the sperm thing, i guess no matter where you cum (unless she swallow every drop) there will always be some left. Even if you don't cum in the condom, there is surely some pre-cum. So well we shouldn't be too worried about it. I supose you can always bring a ziplock and store the condom there once you done if you feel very worried. Like i said there will probably be some of your DNA elsewhere via other things but thats a part that would be covered.