The river Rede flows through Otterburn in its upper reaches. You should be able to catch brown trout there once the season starts later this month. Towards the back end of the season, from summer to the end of October, salmon and sea trout will be making their way up there when the water is higher after rain. I don't know the river there well enough to guess how far up they will get.
Try a little fly rod for the trout, they're obliging enough to give even a novice fly fisherman a chance and enough interest to learn to catch the bigger ones.
If the salmon or sea trout get that far up the river, try fishing when the river level has risen and then the water cleared to a peaty brown colour. The fish can grow big, with 20lb fish not uncommon. A spinner or fly will get their attention.
You’ll need a rod licence for trout or the more expensive one for salmon and sea trout. Check the local bye-laws to see what methods are allowed as float fishing or bait may be prohibited.
If your mam wants to advertise the fishing as a benefit offered to guest at the hotel then she should have a look at the Fishpal Tyne website. You’ll find more information about the river there too.