While your friend has capacity, only she can make the decision whether to allow daughter's partner to take over. If she didn't have capacity it could be reported to the Office of the Public Guardian and they would take action through the Court of Protection. The CoP only has jurisdiction if someone does not have capacity.
Unfortunately, your friend probably ought to be told of the position, since she ought to be allowed to make the decision as to whom ought to deal with her finances in the absence of her daughter.
The OPG might be helpful in these circumstances.