Will washing child seat belt straps with baking soda and hydrogen peroxide make them unsafe to use?
I have seen some companies that refurbish child seats, perhaps a new strap kit would put your friend's mind at ease.  

If the straps were not soaked for hours in concentrated H2O2 and I saw no damage and could not cause them to break with my strength (healthy adult) I would have used it with my kids.  The baking powder or soda should have no effect.

We bought a used seat for the next size up and have no idea of how it was washed.  Would do the same again.

Stay at home, spend the coin, take out a loan or live with the risk, those are the choices. There are other risks in life like not being sure that the excavator operator is of sound mind and will not swing the bucket into your car at the stop sign, does this stop you leaving the house?

EDIT:
It is possible the dangers of washing are circulated by the seat refurbishing and manufacturing industries to increase sales and not safety.  A car seatbelt can last for decades and sees a lot more use and has no washing warnings.