In your sentence the word "lending" means that they have already given you their boots to use and you are returning them. You could say "thank you for your offer to lend me your boots..."

Hi If someone offers to lend you something but you decline the offer, then you should technically say "Thanks for offering to lend me your boots, but it is not necessary." or "Thanks for offering to let me have your boots...", or as you wrote "Thank you for offering your boots." I would only use 'lending' if the 'lend' had actually taken place(even if you don't actually use the thing). For example "Thanks for lending me your boots, but it didn't rain so I didn't need them." P.S. Remember that 'letting someone have something' can also mean a permanent situation (whereas lending is always temporary), so if someone says "You can have mine." it's important to clarify whether they mean permanently or temporarily (if the context doesn't make it 100% clear)