I guess I'd proceed with caution but go along with the agreements the two of them have made. See, when I hear "trial run," I think to myself, "Uh oh, her husband's not really sure he wants this, he is only testing it out once or twice to see if he can stand it." And he may have quite a struggle learning to "stand" it. Lots of guys do the first time they attempt to share their wife, especially in such an intimate way (as some could argue that sleeping together is more intimate than sex). So sorry to be a stick in the mud, but try not to get your hopes too high up there just yet. Sure this might lead to more nights together, or it might lead to him saying, "I can't do this," and then you'll have another long wait ahead of you before you can sleep with her again (if it indeed can happen again).
Is there a chance this could lead to a regular schedule of you/her sleeping-together nights? Indeed there is, which is super wonderful. But in the meantime, all bets are off. We do not know how her husband is *really* going to react to his wife sleeping even one night with another man. Could turn out fine, could melt down into a roiling pit of drama.
I'm sure going out with them will feel awkward at first but I'd encourage you to do it anyway. You need to learn to get comfortable around her husband (and vice versa), a little at a time.
No special advice other than that.
Love means never having to say, "Put down that meat cleaver!"