Also, if she is breastfeeding this would make her dry and the friction could burn when urinating. A lubricant would help with this. But first make sure there is no infection.
My first guess would be a urinary tract infection which would NOT necessarily be picked up at the 6 week check up. Even if she IS still sore that wouldn't explain the burning sensation....I think she needs to go back to the doc and explain these symptoms.
did she have a natural?? Because after 6 weeks she still may be sore and her stomach could still be contracting back to its normal size!
Sounds like it could be a urinary tract infection which is easily treatable. she should speak with her doc about it and they will be able to diagnose.