That is just so hot, I love knowing my wife has another mans cum in her and absolutely love to witness the moment he cums in her. But to actually feel him cumming in her takes it to a whole new level, will definitely give it a try.
Do that, you will love it. So will her lover if you press searching and skilfully enough with the pads of your fingers----two to three is optimal---and lightly stroke the midline skin-seam running forwards from his butt-hole and over his pump-bulge. Your pressings will also serve to focus him on and (when he gets to ejaculate) greatly increase his awareness of and greatly amplify the already delicious sensations of his orgasmic throbbings and spurtings.
If you haven't already, practise it on yourself first in private with you lying on your back and thighs widely splayed to give your free hand easy access to your pump-bulge.Then when you get to do it on your wife's lover you will be more aware of the significance of the appreciative and urging sounds he lets out while you are indulging him, especially when he's building to orgasm.
But be aware that with all but a few men the pump-bulge, along with the butt-hole, is for them the most private and vulnerable part of their genital anatomy, so you would do well to obtain the tacit permission of your wife's lover before you work your magic on his pump-bulge with your fingers.
That said, some men, even young ones like my buddy, when they're fucking the woman in a threesome, will by cocking their bums backwards and opening their thighs wider during thrusting, positively invite their pump-bulges to be caressed by the woman and/or the second male in the threesome. If that happens, you can be sure that the guy is really and truly up for it and perhaps even subtly signaling he desires to bond with you homosexually---which, take it from me, can turn out to be a beautiful bonus.
In full-on MFM threesomes such profoundly erotic opportunities abound. All you need do is man-up and take advantage of them in the very act, then everybody wins.
But remember, in Love as in War, timing is everything.