I have an old friend like this. I’ll call him Jack.
We were mates at uni, 30 odd years ago, not that close - we just shared several nerdy interests so were forever at the same events. We would have lost touch pretty much straight after graduation, I imagine, except one of his closest friends married one of mine - so we (& then with our respective spouses) used to run into each other a couple of times a year at our mutual friends’ parties.
Come mutual friends’ joint 40th, a villa holiday abroad for a dozen of us, I was recently widowed, he was recently divorced.
By this time I’d had years to notice that the quiet, hard to get to know, spotty anoraky lad I’d known at university is actually incredibly clever, seriously funny & an all round good egg.
We had what we fondly imagined was a terribly discreet holiday fling. Our mutual friends were absolutely delighted, having clocked us within days. I was up for seeing how things developed; Jack was more ‘well I had a huge crush on you back then & I think you’re great & I’m having a lovely holiday, but…long distance…kids…too complicated…we’re friends having a fling’
Absolutely fair. We did end up FWBing for a year or so because work coincidences meant we were unexpectedly in the same neck of the woods every few months. Then I moved further away & called time. Completely amicable on both sides.
We’re still mates, & nearly 40 years of friendship is much much more important than the fact that we spent a few months having occasional sex at a time in our lives when we were both having a tough, lonely time of things.
If I were your girlfriend, I’d tell you you were being ridiculous. Absolutely I’d go for lunch with Jack if I was visiting his city.
If I found myself in a relationship with someone who was mistrusting my ability to go to lunch with an old friend without shagging them, it wouldn’t be the friendship of several decades I’d be querying, it’d be CBA with the new boyfriend & his insecurities.
As for the ‘telling him in person’ thing, I get that from her POV. For me it would be in order not to make a big deal of this news. A ‘btw, let me tell you about my new fella’ message could come across as ‘…because I’m making it crystal clear that I AM SEEING SOMEONE & not offering you a bunk up for old times’ sake thank you very much’
I’d be much more likely to let that come up naturally in conversation over lunch, than make it an ANNOUNCEMENT.