I'd just say "Sorry, I haven't got anything available, so sorry I can't help out - but I heard Aldi/wherever is doing a lot of ski stuff cheap right now, hope that helps!"
Having said that, the amount of people assuming that the 15 yo will damage/lose/break things is crazy - if the 15 yo was generally careless and didn't look after things, sure I'd feel the same, but if they were generally responsible then I would be happy to lend things, perhaps with the proviso to your friend that you'd want the stuff replaced if it got damaged.
"Hey Friend! I actually do have all my stuff, and your DC is more than welcome to borrow it, but I want to be up front with you - it's quite expensive stuff so if anything gets damaged/lost I would be asking you to replace it as I can't afford to lose out that money and it's not in my budget to replace it right now. If you'd rather try and get stuff from other people I understand, otherwise let me know if DC want's to come round and try stuff on to see if it fits."
I get the whole not lending/borrowing things but when it's something like ski gear I totally understand the rationale - why shell out a lot of money for ski gear if your kid goes on the trip, hates it and you're left trying to sell on/get rid of a load of stuff that will never be used again? Equally, if your family doesn't ski a lot, perhaps they will only ever go ski-ing with school and if you have family/friends who have spare kit they don't mind lending out, as they ski more regularly, why spend that money for the sake of it? They are not unreasonable to ask, providing they are willing to accept a "No, sorry" as much as a "Yes, of course" response.
Whenever any kid in my family has started anything - whether it be a new sport, club, group or even instruments - there's always a message going out saying "X has decided to try . Since we don't want to spend a load of money before we find out if they will commit to it long term I was wondering if anyone has we could borrow or knows anywhere we could buy second hand/hire them for a few weeks till we know if they'll actually stick to it. Thanks!"