You can give your customer Product Manager rights in a specific product. This will allow them to create simple queries in a specific context.
If they require complex queries - it can be solved with the additional group (https://www.ptc.com/en/support/article?n=CS35281). If you are using Windchill 11.0 M30 CPS 12 or later (or planning to update to it) - I would strongly suggest creating this group at Site level.
