Why might this person have bruises ?

Blood thinners.

Since the doctor put me on them I bruise like a peach, and sometimes in places that are impossible to bang/knock.

I am clumsy AF and bruise really easily, which is a very poor combination. I always have corking bruises on my arms and legs, and a few times DP has been given funny looks if I’ve had on a short sleeved top or whatever. I literally walk into things in our home that are static (we’ve lived here 10 years) and am constantly bashing my hips off desks at work (teaching).

My upper arms are covered in small bruises from insulin injections - just the front and sides rarely the back

I bruise easily and always have. I regularly get bruises and have no idea how I got them. I have had a few people make comments and never take offence. Mainly they just say on you have been the wars or a similar comment.

If she was in a long-term relationship with someone, then you might have cause for concern, but if she's enjoying casual sex with a few different men, it's not like she's trapped in an abusive situation she can't escape from. It's a very different scenario from someone living with an abusive partner that they're in love with and/or can't escape from. It's far, far easier to walk away from someone who is harming you if you're not in a relationship with them and have other partners.

FWIW I regularly have bruises on my thighs, which bruise easily. Some of them are from sex. Some of them are from workouts, gardening and the corner of my kitchen table.

Many years ago I was a victim of domestic violence. I never, ever had bruises on my arms and legs from that. The only time I had any marks that anyone would have seen were when I was hit in the face. Other than that, my injuries were things like cracked ribs and broken fingers, not bruises on my arms and legs.

I bruise easily. I also used to do ju jitsu

I was a victim of DV.

The only bruises on show were from ju jitsu, the ones from DV were not on show.

I would ask your friend but I would not push it.

I'm covered in bruises and scrapes most of the time. Sometimes have a black eye, a limp or finger grab marks on my arms. Once or twice a year I have a cracked bone or a concussion or similar.

I do a martial art so it's expected. I am careful to tell people though so DP doesn't get accused of beating me!

Does she play sports? When I played a lot of sports I always had bruises on my arms

I bruise my upper arms from the sledges at the gym

Ok, slightly TMI time.

DP and I are fairly vanilla, but she does enjoy being restrained a bit during sex. We're not in tying fancy knots territory, but she likes it when I trap her legs open with my legs, or if I basically pin her by her upper arms during missionary.

Unfortunately DP is also very pale, and bruises like a peach. The bruises you're describing @0livesandcheese sound a lot like what DP gets sometimes.

So it could be nothing, but then again it could be something. So I'd just ask her. Sounds like you're good friends so how embarrassing is it actually going to even be if it's a sex injury?

I would casually ask. I did this exact thing a couple of week’s ago. Friend had a large bruise on their arm. I said Oh do you know you have a large bruise (sometimes people don’t realise). She did know and it was something to do with a medication she recently had to have. (She was quite seriously ill recently).

Is she diabetic? If so, she may inject into her upper arms, which can causes finger size bruises. I am covered with them on my stomach and thighs, in various stages of healing due to injecting 4 times a day.

I have bruises like that all over my arms. I’m a support worker and one of the clients is very grabby plus I bruise very easily. I see people giving my husband the side eye when we’re out but I do explain.

If it is from sex, she likely won’t be embarrassed.

So that comes down to whether you would be or not.

Personally I’d ask and deal with the embarrassment if it was, because that’s better than not asking and it potentially being something more sinister.

I would just ask about them and if everything is okay

Why do they have to be from sex?

I would have asked whether she’s a Kendo practitioner (although that’s usually the wrists)…

Pets, medical conditions and yes (potentially) consensual sex would come to my mind as well. But I would definitely ask a close friend.

My upper arms and thighs sometimes bruise badly with no known cause. They just appear seemingly overnight, and like a PP, I sometimes end up with scratched and British’s from playing with my over enthusiastic dog.

Realistically, if you asked and she said someone was hurting her. What would you do?

Or you could get an in-depth report on her sex life. Either way, you won't know what to say.

Nobody other than her knows it’s that though. It can also be a sign of abuse and someone checking in could be what she needs to open up.

it’s always best to check these things than to just assume all is fine.

It is a classic domestic violence bruise as well.

Hard to lighten up when I’ve worked with abused women.

Yes, but she may not have been a willing participant.