Thank you for the extra information.
While it's soon enough after ingestion to induce vomiting, I don't think the risks of doing so outweigh the benefits at this point.
Typically severe signs of phentermine toxicity start at about 5 mg/kg. That means a dog Duffy's size (20 pounds = 9kg) isn't expected to have severe problems until he's had more than 45mg.
When a 20 pound dog receives more than 45mg of phentermine, Central Nervous System signs tend to predominate: agitation, tremors (unlikely to see seizures
at this dose).
Phentermine has a pretty rapid onset of action, typically within 2 hours. So if after 3 hours Duffy isn't showing any symptoms, you may not see much of anything to worry about.
If you'd rather not take any chances, I'd advise taking him to an ER right away so they can induce vomiting, but - as I said - that's a judgement call and not without risk.
Also, if there's a chance he ate more than one 30mg tablet, I'd advise seeking emergency care.
I hope this offers you some peace of mind.