Yes, do something.
But (your “mate” should) consider seeing a physio as well. Hip back issues can be difficult to DIY diagnose and fix because several different things can cause the same problem, and the solutions can be the exact reverse.
e.g. short hip flexors, maybe they need a stretch and to work on some glute strength to pull them out a bit. Or maybe they’re actually just weak and compensating for that by remaining short where they can maximize their limited strength, in which case you should be strengthening them.
A bit of yoga (probably) won’t do any harm, but there’s plenty of videos titled something like “hip strength and mobility workouts” which might be more specific.
For “core” strength just KISS, work round it logically, do supermans (lie face down arms at your side, and pull your shoulder blades down, tense your glutes to lift your feet up until your shoulders and legs are off the floor, hold as long as possible (30-40s max, the benefit of doing longer runs out quite quickly past 20s) rest and repeat for 5-10s more in total each day. Once you get to about 5x30s, make it harder by doing a superman pose with one or both arms to add leverage.
Repeat for a plank (do it strictly with your back set like above, a deep breath, rotate your shoulders down and back, tuck your shoulder blades in, tense your abbs in and glutes to maximize abdominal pressure). The idea isn’t to have a conversation and hold it for hours, it’s 20-30s of really hard work to build strength. Have a read up on how lifters que for deadlifts or squats. it’s a similar principle.
On your back tensing your abbs lifting legs and shoulders off the floor.
And on each side.
If you have access to a gym, use the dip station to do leg raises (start with knee raises), pulling up from your core, and progress from there to to doing full leg raises from a dead hang.
And stretching to open the hip joints up a bit.
