What to do if my wife is 7 weeks pregnant and works full-time as a dental assistan but her the manager told her she will have to keep taking X-rays?

If one of the responsibilities of a dental assistant is to take X-rays and the job cannot be done without taking them, and also the nature and staffing of the office is such that the employer cannot reasonably have someone else do the X-rays while your wife does other dental assistant tasks (that is, for example, she is the only assistant; there are no other assistants who can do the X-rays while your wife does other things), then yes, she can be fired.

An employer must "reasonably accomodate" a pregnant woman. A reasonable accomodation is one that is not too disruptive or expensive to the employer--such as, if there are two or three assistants in the office, having another one do the X-rays while your wife does non-X-ray tasks. However, an employer is not required to do anything unreasonable, like hiring a new person to do the X-rays, or having the dentist him/herself do the X-rays (that's why the dentist has dental assistants), or having staff who lack the proper training or credentials (like an officer manager) do the X-ays. So if your wife cannot do the job of dental assistant without doing X-rays and there is  no reasonable way to have other assistants or technicians cover for her, then she can be fired.