Couldn’t agree more. This is a general fact of life; pushing someone into receiving help they don’t actually want can so easily backfire and cause a lot of misery all-round. It can even make things worse if subtance abuse is involved (the ‘I’ll show you’ effect).
I also think it’s important for someone who knows a lot about ADHD (i.e. a medical professional) to have their say, as a layman’s diagnosis is rarely accurate. It is so easy to get an incorrect or even half-correct idea about a disorder like ADHD, make generalizations, and then jump to conclusions. Your friend may not have ADHD at all! The same thing happens with conditions like Schizophrenia. A lot of people think Schizophrenia is synonymous with having “split personalities”, when that often isn’t the case at all; the condition more often presents as an inability to stay on one train of thought (loose association), eratic movements, and disjointed speech. Laymen don’t diagnose people for a reason!
You could try mentioning to your friend that they have some symptoms you believe to be common in people with ADHD, but to just proceed as though you have definitely correctly diagnosed him is most probably a mistake.