There is a lot of coaching equipment available these days, most of it very reasonably priced, and it is well worth you and buddy/training partner looking to invest in some, but the reality is that you already have the best (and most expensive) piece of equipment to help you: your smartphone, or to be precise the video function.
Get into the habit of using your training partners phone to vode their practice. Check with them exactly what they want videoing, review the video from previous sessions before you start so you know what you are looking for in their training session, and review the footage together at the end of the session to discuss improvements and areas to work on next time.
I would also highly recommend that you are videoing correctly after the first couple of repetitions of each drill so that you don’t end up with unuseful footage at the end.
A tripod may well be a good investment between you as some drills will require two hands so videoing may be a tad difficult.
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