The interest rate will be on the high side- maybe 6% at best so if they borrowed say £100000 that would cost you £60000 over 10 years. It would clearly be a lot more if they borrowed nearer £400000. However in my experience they will not lend anything like the full value. There will also be set up fees, legal fees, possibly early repayment fees, and they can place conditions such as you must do regular maintenance, decorating etc. It's a difficult area so I would pay for financial advice first.
Rather than equity release couldn't they just remortgage over say 10 years?