My sister in law has had a kidney transplant a year ago now she is doing well. But can she now go in for a tooth extraction which has been advised?

Yes. She must take full sanitary measures for the extraction to prevent an infection if she is still on immunosuppressants.
Yes. Check with your doctor and/or transplant coordinator to make sure you follow the latest guidelines regarding prophylactic antibiotic use.
Yes. Most often she will need to receive an antibiotic just before the procedure. She should discuss this with her transplant team.
Antibiotic coverage. People with organ txps + suppressed immune systems (from the anti-rejection drugs) generally require all health maintenance care, including dental treatment. It's more important to treat infection earlier + more aggressively than in other people. If a dental extraction is required, it should probably be done in conjunction with antibiotic coverage. Should definitely ask txp md. Tell the dentist!