I'm not sure if you can just shrink the waist, you could try boiling a large pot of water and dipping the waist band into it...but I'm not sure about that.
Alternatively, take them to an alterations place. A friend of mine...
Eat when you are hungry, stop eating as soon as you feel satisfied, not full, drink lots of water, up the amount of exercise you do and make a few obvious changes like cutting down on some (not all) fat and sugar.
If you are not that close, I would suggest buying a denim wallet to start with (of course with some golden coins inside...!). Jeans or clothing is perhaps a bit personal for someone you don't know very well. People have their own...
I read somewhere that sometimes dandruff is caused by a fungal infection, so if all shampoo treatments fail, you could always rub some fungal infection cream into your scalp and see if that helps (i know this sounds weird, but it works!)