tell him/her to accompany you to a dentist for your own checkup and while at it suggest to them to see the same doctor after you are done. Make sure you explain this to the doctor in advance

Google cure for halitosis. One major cause is cavities and may need deep cleaning by a dentist and possible extraction. Mouthwash mostly masks it but does not do the trick. Flossing is very important too along with brushing teeth 2x/daily.

Update?

There are many diseases that lead to bad breath and body odor, among them cancer. A little compassion and empathy would go a long way.

Major culprit of bad breath, where dental hygiene is met and loss of sense of smell has occurred, are the sinus. Among the diseases affecting sinus are
1. Sinusitis (with constant post nasal drip)
2. Rhinitis (There are several versions of this disease)
3. Diphtheria
4. Tuberculosis of nasal mucosa
5. TMAU