How to tell speakers that their English is terrible?
I think that your plan is based on number of possibly wrong premises. And even if they were mostly right, I doubt such a plan could have any reasonable success. Let's see my reasons.


  I just had a lecture from someone who has been a senior scientist (and has completed a PhD, post-doc) at a hospital for already 15 years.So I'm assuming this person is experienced in giving talks in English.


I've met along the years many senior researchers, even native English speakers, who where definitely not experienced in giving talks, at all. If academic researchers are frequently experienced speakers because, at least, they have teaching duties, some researchers from research institutes are really "lab rats" who rarely deliver talks. To give you an example, a few years ago I was attending a poster session in a conference and I told one of the presenters (a native English speaker) that I was surprised that his work had been accepted as poster presentation and not as an oral one. He told me that it was actually accepted as oral presentation, but he asked for a poster one because he doesn't like to deliver talks.


  My impression is that the talk was a waste of time for the two dozen people present.


Talks can be a waste of time for other reasons than terrible language: slides with unreadable results, talk targeted to the wrong audience etc. I certainly attended many talks that could be considered a waste of time for almost anyone in the rooms, and a large fraction of these talks were delivered by allegedly experienced speakers.


  Now I wonder if the speaker is aware of this problem, my guess is no


In my experience among non-native speakers of English, most of the people is well aware of their level of English and of their pronunciation: your guess is likely wrong.


  I feel the need to bring this to the speaker's attention.


Would you feel the same need if the talk were a waste of time for any other reason? And note that preparing readable slides is usually much easier than fixing pronunciation. And are you planning to do such an action for all the useless talks you will attend in your life? 


  I feel the need to bring this to the speaker's attention.


As I said, the speaker is probably well aware of this issue, but can you offer any solution that the speaker is not already aware of? 

Improving pronunciation is not something that can be done easily, especially if one is not keen on languages. The speaker might have had good reasons for not having been able to improve pronunciation more than that. Time and money can be two of them. I don't know how many languages you speak -- you speak more than one, right? I speak Italian, my native language, English and French. My level of English is decent, but my level of French is basic. I cannot make great conversations in French, just short sentences, and sometimes I've been misunderstood (e.g., I once asked for a book called Rue des Maléfices and the clerk searched for Roue des Maléfices). I'd love to improve my French pronunciation because I spend most of my vacations there but, guess what, I really cannot find the time to do this. Or I'd like to learn German, because I have many German colleagues and I also found some nice physics book in German that I'd like to read without Google translator. Again, I cannot find enough time to learn German even at a basic level.

To conclude, don't.