The OP has already decided to tell her husband about the past.
Therefore whether to tell him or not is irrelevant now, as she has decided.
The area still under debate is what to do if he is not happy about the friend coming to stay. There is no suggestion that the issue will be that OP resumes relationship with friend, because somehow being in the same house makes them unable to resist each other. There is no suggestion that husband will hit the roof when he hears about friend being former xexuql partner.
If husband is genuinely fine about friend coming to stay, then there is no dilemma for OP.
There is only a dilemma if husband is uncomfortable about it. Minty expressed a mans view and would have had difficulties with it. Lots of people have said that we simply cannot know how husband will feel. There is no right or wrong feeling about it...it is just a feeling. So posters saying he has no right to feel aggrieved, are right that he has no right to feel put out that there is a sexual past, but I think that if he doesn't want to be confronted with that in his own home, he DOES have a right to express that feeling.
The OP can then effectively say, that the friendship is more important than husbands feelings and she still wants friend to stay, citing reasons such as he has booked the flights,mor is an old friend. This is to disrespect husbands feelings. Or she can say, okay she will ask friend to stay elsewhere and see him outside the home. This is to respect husbands feelings.
This is not just like having different views about whether to order curry or Chinese. Some posters seems to see the dilemma as the equivalent of this in its insignificance. For some people, coming face to face with a sexual past, they theoretically accept IS difficult and there is no need to force our husbands and wives to confront it if they would rather not. To insist on the friend coming to stay or to pressure the husband into agreeing to it, when he is uncomfortable, could be damaging long term. And that damage is so easily avoidable...so why do it??
For husband to say he is uncomfortable is NOT to disrespect OPs desire to see her friend. The OP can still see her friend. It would not be the equivalent of OP insisting on friend staying.
So the choice (if husband is unhappy about friend staying, which he might not be anyway) is simply to prioritise own desires, recognsing that will hurt husband, or husbands feelings.
Choosing husbands feelings is not suggesting her past is something to be ashamed of, or that she and friend can't be trusted in same room. It is just saying that husband is now the key relationship and she respects him and is willing to put his feelings first over this single issue. Sends a loud and clear message either way, about the value the OP places on husbands feelings.