Hello Rose,
Welcome! So I searched a bit and on one community Q & A site, a nurse wrote in with info about something called Oasis Wound Matrix. You should Google it and speak with your mom's doc about it. It is expensive but seems to help with wounds that won't heal. Also, there are different foams you can use with the Kalypto vac that aren't the silver based ones. Again, don't hesitate to consult with mom's doctors.
The pred itself doesn't technically affect wound healing but because it is immunosuppressant, her body could take longer to heal.
So while I don't have any personal experience with delayed wound healing, there seems to be options out there. Please speak to her doc about the silver sensitivity and using different foams as well as the Oasis, if he thinks it might help...and you have the financial ability to cover the costs.