Hmmm...that's a tricky one.
With it's excellent year round climate most Tenerife hotels don't tend to have indoor pools and if they do it's normally part of the Spa and extra charge.
However I'm sure I remember one at the H10 Costa Adeje Palace (which has an all-inc option) and it was open to all when we were there a few years ago.

Thanks for that. She wants an indoor pool because most people we have spoken to say it is too cold for the sea and the pools at the end of October.

We've been to the Canaries 12 times now and I'm ashamed to say I've never been in the sea, so can't comment on the temperature at that time of year.
But I've been in plenty of outdoor pools in the Canaries in winter and they are a very pleasant temperature. I can see that by October somewhere like mainland Spain is cooling down rapidly, but the Canaries are still very warm (as they are pretty much year round), so I'm not sure the advice she's been given is altogether true.

Pool temps do vary from hotel to hotel though. We were at the Bahia Principe Tenerife recently and the heated outdoor pool was pleasantly warm. We were then moved to the Playa La Arena due to the ash cloud and the "heated" outdoor pool was noticably cooler.

All depends on your definition of cold, I guess.