look at budget and size.
start with size (sorry if I’m teaching you to suck eggs but someone might find it useful)
binoculars are usually expressed as X*Y, – as you quote above his current set are 8*25. this means they provide 8* magnification, and have an aperture of 25mm at the far end (the objective lens)
8* is about right for birdwatching – it usually provides reasonable detail / magnification while keeping a good field of view.
Aperture wise, the bigger the better – a bigger objective lets in more light, so will provide crisper pictures and better colour. 25mm is normal for compact binoculars, it’s on the small side in general, but is acceptable if the glass is good quality. “full size” binoculars tend to be 42mm objective and the difference is really noticeable
If he doesn’t move very far then for a given budget, bigger binoculars will probably give much better image quality – but if he wants to take them here there and everywhere, the bigger lenses will be noticeable as they will be much heavier.
the other consideration is budget – how much do you want to spend. £100 will get you a reasonable set of compact binoculars. £2000 will get you a fantastic set of full size binoculars
so, what’s your budget?