I won't repeat what others have said as clearly you are trying to do the right thing by these ponies. Not impressed by the owner trying to make this your responsibility
.
I think if the owner does leave them there they would effectively be abandoned animals, the below link is good but effectively they would be your problem to sort out, you could try and reclaim costs from the old owner but possibly a lot of time and effort to do so...
www.bhs.org.uk/advice-and-information/the-law/abandonment-and-fly-grazing
In terms of how much time it would take to look after them, probably at least 20-30 mins a day in summer (quick catch and check over daily, top up water and weed and poo pick the field), more in winter (all above plus change rugs if they wear them, give hay, break ice on water). As others have said you will need to either pay someone or allocate more time for field maintenance, harrowing, dividing up the field, fence and hedge maintenance etc).
Another good link to give you some idea of costs:
file:///C:/Users/ellyd/Downloads/The%20Cost%20of%20Keeping%20a%20Horse%20or%20Pony.pdf
A final thing to think about is that horses are herd animals and shouldn't be kept alone, so you would need to think about what happens if/when one of them dies - the other will need a companion so you would have to think about getting another (thus ending up in an endless cycle of ponies!), finding him/her another home (not easy for en elderly pony!) or PTS...