"You sound a lovely considerate husband so long as you can turn a blind eye, I may get flamed but you also sound incredibly wet.
You’re concerned for your wife and you have doubts, yet you won’t accept that you need to deal with that doubt in any way, shape or form.
You don’t sound like you want to approach her about it to discuss anything at all. Even if the best way to do that may be to question it financially, and go from there.
You don’t want to investigate what’s actually going on and to be honest I think you make excuses because you don’t want to know the answer."
This. In fact, I'd go one step further and say you are letting yourself, your wife and possibly your children (if you have any) down by allowing this situation to continue without challenging it. It's not some huge shameful wrong to mistrust someone if they are behaving in a way calculated to undermine your trust, nor is it a huge shameful wrong to act on that mistrust by e.g. following her.
Something is very wrong here. You know it is. Personally, I think the most likely explanation is that appropriate and healthy boundaries have been completely destroyed and your wife, at a vulnerable time, has been persuaded to feel like she cannot manage without this therapist. He, in the meantime, is probably getting off on this in all kinds of unhealthy ways. She has clearly been too deeply sucked in to be able to see any of this clearly. But if this process were actually assisting her, she would not need this level of intensive therapy three years on. So it's clearly not helping her.
I think your wife is at risk from this person and it's time you became more active in working out what is going on and seeing how it should be addressed. It may be that the thing is hopeless and cannot be fixed. But you should at least try. For me, this would include asking her about it and possibly also following her. Yes, she may be having an affair with this man or with someone else. But if she's having an affair with him that is completely wrong and is an abuse of power on his part. She needs to be protected from that if she possibly can be. If she's having an affair with someone else, then you will at least have a better idea of how matters truly stand within your marriage.
In a situation like this, there comes a time when you need to make a stand to protect your marriage and your family. I think that time is now.