"So he stopped loving and stopped wanting to be with his wife and has left her. Do you understand that he could do this to you too?" and people wonder why it is that so many have affairs in order to leave marriages when the attitude is that they shouldn't be able to leave because they no longer love someone? 
OP, as he's been separated now for over two years the unreasonable behaviour clause no longer applies and he can apply for divorce on the grounds of a two year separation. She could still contest it at this point, but if she does then you will need to wait another three years until he is legally divorced.
I do actually sympathise with the point of not wanting to file on the grounds of unreasonable behaviour and to want to wait for a clean no fault divorce, but now that two years is up he is in a position to apply for one, and I would be expecting him to do so at this point, even if it means she contests it.
Also it would be possible to factor in the joint mortgage in the consent order, so that only his property is taken into account in any future changes.
FWIW very few couples wait until their actual divorces go through before starting to date/see other people, as divorces where there are children and/or finance involved take a significant amount of time. Mine took nearly a year to go through, but we had a separation agreement drawn up which took us through the process and took account of things like the finance and maintenance so that I could move out of the family home and into my own place etc.
My DP and his ex didn't want to go the unreasonable behaviour route either for religious reasons, but truth was that the marriage was in trouble for a long time before he actually left, but there was probably some fault on both sides iyswim. And she originally said that she would never grant him a divorce, but she did actually file for no-fault after two years and it took a matter of a couple of months to go through but there was no money or real assets involved.
The things you need to look out for are:
Protecting your own property and your financial interests to make sure that it can't be counted as a marital asset.
Is he still living in the marital home in which case you need to be extra cautious. My DP had actually left the fmh at the point of separation and hadn't seen his ex at all at the point of divorce. If he's still living with her though this should set alarm bells ringing.
And if he doesn't want to divorce her at all this should also be an issue, as while the unreasonable behaviour bit is understandable, wanting to stay married on religious grounds but still living with someone else really isn't.