To the best of my knowledge your husband can send DPAs on his mother's behalf in this case. If you want to be absolutely certain you could give the Information Commissioner's Office a ring on 01625 545 745.

I would amend the template letter / e-mail to make it clear that the notice is written by the PoA.

If your mother-in-law is being sent junk mail by lots of different companies it's worth asking the senders to let you know how they obtained her personal details and who they have shared her details with. It very much sounds like her name is being traded by list brokers, in which case you'd be in for a game of whack-a-mole. By finding out who the list brokers you'd get to the source of the mailings. (It's unlikely senders will give you such information but it's worth a try).

You can also registering your mother-in-law's details with the Mailing Preference Service. The opt-out scheme for personally addressed junk mail isn't anywhere near as effective as data protection notices but it should also prevent list brokers from selling her personal details.

Finally, if the letters are scams (rather than legitimate marketing letters) the situation is much more complicated. Scammers won't take any notice of data protection notices and the Mailing Preference Service. The best you can do with scam letters is confiscating them before your mother-in-law sees them (perhaps by redirecting her mail) and reporting the scams to Action Fraud.