I’ve heard this many times since the rise of Facebook, old ex’s searching out previous partners. Very few have survived it’s the thrill and excitement but it rarely lasts. Out off the ones I know of, only one lasted, and they literally ran away to a new life, new house, new jobs new area, so no really knows if they even survived. Lots of advise already given. My advise to you would be, even though it’s early days she needs to prepare herself for how she wants to handle this if and when he comes crawling back. Of course she’ll want him back with open arms as she is hurting, however it’s a long hard road after infidelity and there will be many problems on a very bumpy road. She won’t trust anything he says or does and he will need to give her a lot of reassurance, this alone puts pressure on an already fragile marriage. At this early stage just listen and dont have any judgement on him. Sorry for loss of your husband.