Oh it's very possible to go higher than that - mine hit something like 240 over 180 once!
What's defined as "dangerous" and needing a hospital visit is when the top number is 200 or above.
170 over 110 is definitely high though and definitely requires a GP visit.
That being said, if he's experiencing anything like prolonged headaches, shortness of breath, palpitations or similar then emergency treatment should be sought.