Apologies for being a little late to the party. I live near Aviemore, which is in the centre of the Highlands, about thirty miles from Inverness/Loch Ness.
>What clothes should they pack, especially for the Highlands?
Temp here could be anything from 50 Fahrenheit to over 70, plus it can be windy. So usual T shirts, shorts, jeans; a fleece and at least a waterproof top is also wise. One or two(?) long-sleeved T-shirts will be useful for 'midgy' areas (almost any countryside ).
Midges can be a pain almost anywhere here. I'm guessing they're on a coach tour of some type? I'm guessing they're headed for Edinburgh or Glasgow first? If they have a free day/half, find an 'outdoor' shop and buy a couple of midge nets or hats - fit over the head and neck, tuck into the top - shouldn't be expensive, work better than constantly plastering midge repellent over face and neck. If either finds they suffer reactions to midge bites (big red lumps, very attractive ...), some anti-histamines are available over the counter (no doctor's prescription needed) in drug stores (called "chemists" here).
>I think they will be at Loch Ness on Monday or Tuesday.
That's a pity, I'm headed 'down south' this weekend. Nevertheless, I'll PM you my cell number, if they get stuck, they can call me, I can put them in contact with whoever.
>waterproof walking shoes for cities, stout ones for the Highlands.
We have paved roads and sidewalks here. If they go walking, unless they're hiking up mountains, there are lots of well-made paths (the National Park is big into encouraging walking and cycling).
>Like everywhere there are great and not so great areas.
Up here, even in Inverness (the city here), two reasonably-sensible young women will be safe even at night. Outside Inverness, any town or village will feel almost deserted at night, but there are some older people that still don't lock their doors.
Doesn't mean they shouldn't be aware of their surroundings, tuck bags under tables when eating or drinking, etc. but it definitely isn't as concerning as the bigger cities.
>The legal drinking age in the U.S. is 21.
18 here; if they'd like to have a drink, wise to carry some ID that shows their birth dates.