I have a mother-in-law who is 46 and takes thyroid pills since 2009 she also has bad back problems and she wants a bby with her bf is that safe forher?

Yes . Modern medicine is pretty good and she may have a baby safely.
Relatively. There is a baseline risk of about 4% unexpected outcomes with pregnancy at any age. If you add 1-2% for her age and health issues, she still can have a healthy outcome more often than not.