How Long ago did he buy the house, and what kind of mortgage product is he on?
Mortgages for buy to let properties genrerally have a higher interest rate than owner occupier, so it may be that the bank are trying to up the interest he is paying.
Re the valuation, again it depends on what type of mortgage he has - eg if he is on a tracher rate with a LTV of 80%, the way property has fallen might mean that he no longer qualifies for theat tracker and the bank would try to put him on a higher rate.
As long as he can prove it was his own private residence for a number of years (and gets his arrears paid off asap!) i don`t think the bank can do anything.