IMO, correctly maintained high tensile fencing is very safe for most horses, particularly if some or all of the strands are hot. (There's some disagreement about that, some horse people really dislike it. I started out disliking it, and after having horses out in it for a while, changed my mind.) The correctly maintained part means ensuring that the tension is correct. I have seen a horse run into high tensile full tilt and have the wire stretch slightly and the horse bounce right back into the paddock. You certainly don't want it loose or sagging, you want it tight enough that you have to apply a lot of force to get the wire to give.
I think it's fine for your mare alone.
Turning a foal out in it would worry me. Foals have to *learn* to respect fence, including hot wire, and I'd be concerned about them getting hung up in the fence before they figured it out. I'm assuming that there's enough space between five strands for a foal to get a leg through.
The only thing I can suggest is to fence a small area for the foal using some other fencing inside the high tensile wire.