Why?
Further, 6% is almost 1/10th of the entire company in terms of ownership.
It'd be like buying a permanent seat in someone's minivan, even if they replace the minivan for a lamborghini minivan in the future, for about $100 one time fee and a vague promise of "we'll help you set goals on how to get that lamborghini!"
It's a nonsense offer.
Y Combinator's standard terms are $125k for 7% with an established/trusted brand and deep investor network.
No one should be duped into giving up the same level of ownership for 4% of the capital and (presumably) a less well-known brand.
1) 6% is not almost 1/10. It's almost 1/17
2) It starts with 1% which is 1/100
3) 10k is enough to build an MVP in most cases
4) The "seat" is going to become smaller and smaller as you issue more shares for options and new investors