Hi,
my I suggest you go back to the Physio who prescribed the exercises to let her know of your wife's problems.
It's strange to prescribe exercises and then not follow up on whether a patient can actually do them.
Your Physio will have information on where best to buy a suitable mat and what to use for the heel slides. Usually mats are made from rubber - so slides are not possible unless you use a 'something' to eliminate friction; e.g. plastic bag, double satin sheet.

Cheers,
Fyzzio