I don't know about that particular model Singer but if it's running well - why not use it? Buy the $15 foot .... heck buy a $20 walking foot too. That's a lot cheaper than a $400 machine. Looks like it's a low shank foot.
You say she 'inherited' the machine from the MIL - did they MIL pass away? or will she be insulted that her machine wasn't 'good enough'? How about the hubby? How long has she been sewing on it? a few months? a couple of years?
Buying a new machine is like looking for a new home...you have to know WHAT you want before you buy and you have to know your BUDGET.
Personally I'd just probably buy a few feet for her (check ebay) and let her grow into sewing and let her figure out what she really needs.