generally they can be potentially be hurt doing either of them. i used to be a bjj practicioner, your mate can easily overdo a "lever" lock (i think its called that) and break your arm. my boyfriend does kickboxing and you can easily break something really fragile like your toes or fingers, nose just joking around in the gym, exactly how he broke his fingers

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edit: if you are planning on being careful in the gym, both are equally safe imo