Margaret, your daughter's anxiety is understandable, but no harm can come to the baby from a dental Xray. Only a tiny dose of radiation is used, and focused on one small area. But even mammograms are safe during breastfeeding (though not usually helpful in diagnosis because of the density of breast tissue). Breastfeeding is meant to continue for years, so I wouldn't advise waiting for the Xray until it's finished!
See this link to a US site (unfortunately the NHS doesn't give such comprehensive info) which explains various mistaken ideas about what could harm breastfeeding.
http://pediatrics.about.com/library/breastfeeding/blbreastfeedingp.htm
The site points out that most doctors, including paediatricians and breast surgeons, don't have comprehensive educatin in lactation. The same applies to many midwives and health visitors. For such queries, It's best to ask a breastfeeding specialist such as an accredited voluntary breastfeeding counsellor, or an International Board Certified Lactation Consultant (IBCLC). There's a national breastfeeding helpline: 0844 20 909 20
I'm happy to be contacted through my website about this or any other breastfeeding query.
Alison Blenkinsop DipHE, IBCLC
Media Liaison, Lactation Consultants of Great Britain
www.linkable.biz