If he’s planning a male costume then the basics will be a shirt and hose.
The cheap way to do this is basic leggings, primark, or any of the supermarkets will work, check the women’s side and see if you can find some non black ones. Much more Marcher to go for wine red, brown or even mustard gold if you can find it.
Shirts check the charity shops. You’re after cream or off white grandad shirts, the ones with the little stand up collar. Oversized is better, again check the women’s rails too. If you are careful you can cut the stitches on any breast pocket to remove it, run it through the wash and that should remove the sewing holes so it will look like it was never there.
If you can find them / afford it a couple of sets of both will make wet or very hot events much nicer.
Then you just need a t-tunic, cotehardie or doublet to go over it.
If you have even the basics of sewing knowledge a t-tunic is a very simple garment to get started on, one in cotton/linen for the summer, one in wool for cold events will do you great - use charity shop bed clothes and blankets for your materials, there’s patterns on the Empire wiki.
Cotehardies and Doublets are a little more complex as they need some fitting and buttons or lacing. If you have a friend or relative who sews they’re not that complicated so worth asking. Otherwise haunt the second hand sellers on FB or in the field and you’ll probably find one that works for you.
Invest in your boots - you want waterproof, ankle supporting and comfortable for walking all weekend, so if you have limited budget get those sorted first.
Add a belt, get a cloth or leather bag (charity shop) for your pencil and notebook, find a tankard (charity shop) and then check the jewellery racks in the charity shops for broaches that might work, bead sets etc that will give your character something to make them memorable and talk about.