Sorry to hear your worry for your friend is causing so much challenge for you. You sound like a very sensitive thoughtful person. Your friend is lucky to have someone so sensitive in their life.
From personal experience, I find stress causes a noticeable ramp up in energy which I definitely feel at times, especially regarding the heart. If I don't work it out, it starts to manifest as physical tension. Some ways to work it out may involve massage, breathing exercises and stretching (as Katy suggested), talking it out (aka venting constructively), meditating on relaxing the body and so on. Some people find it helpful to undertake sensory exercises to ground them out of stress. Being fully invested in these exercises (involving sight, sound, smell, touch, hearing) brings their attention into the moment and out of future or past worry. I know, easier said than done. A sensational bath at night by candle light (with scented candles) is a good therapy.
I know this is going to sound a bit trippy but the first thing I do in the morning involves working up my energy. I'll do this through stretching and meditating on my feet and hands until I can feel them buzzing/tingling. Another thing involves rubbing my hands together. You notice people will naturally do this a lot of the time without even realising, when they're trying to work up to doing something, getting themselves motivated. Another thing people may tend to do, in order to get a bit of energy going when they've been sitting down, is stamp their feet. So the feet and hands tend to be a starting point to work up for some folk. Sounds weird but because you're already packed with the energy that can come with stress, you may naturally be feeling 'the buzz' some people look for.
Observing how our body is interacting with our mind is an interesting process. While at times, we can be left feeling helpless as our mind seemingly takes over, looking to the body as a gauge regarding dis-ease (unease) or wellness means we can develop physical ways/exercises for managing our mind, in order to change it or maintain it.
I hope everything works out well for your friend. Again, they are lucky to have you in their life. Maybe you could consider practicing some relaxation techniques with them if they're feeling a bit overwhelmed at the moment.
Take care :)