How do I tell my wife it’s ok to cheat?

After almost 30yrs of medication for Crohn’s disease my libido has gone, totally I have no desire for intimacy and am unable to achieve any sort of state of arousal despite trying every ED medication going. My wife and I have previously discussed the situation as she still at 58 desires sex, we have been sexless for four years.

Recently a colleague of hers ( married) has shown interest in her and she has told me they,ve lunched together a few times. I know she finds him attractive and his name comes up in conversation regularly so I know he’s on her mind.
I’d like to tell her it’s ok to enter a physical “arrangement “ but just can’t find the words. How do I approach this without her overreacting like she can do in some situations?
Grateful for any advice.