If faced with this attempt to put the onus on the questioner then you may tactfully shift the responsibility to him. You might ask him who this professor of chemistry is and what he really has had to say about abiogenesis and early evolutionary events.
When the Witness fails to be able to tell you this then you could respectfully inform him that you know the professor is David W. Deamer and the WTS is paraphrasing information he published some time ago in publications like "Evolution, Investigating the Evidence". You could also perhaps suggest in a kindly way that the Witness follow the suggestion in paragraph 7 of the September 16 WT article "Young Ones, Strengthen Your Faith" which states:
"Jehovah wants you to use “your power of reason” to prove the truth to yourself. He does not want you to believe just because others do. So use your thinking ability to gain accurate knowledge."
You could provide further assistance by suggestion they read the actual publications written by Professor Deamer. If they don't have easy access to these you could make good use of your tablet or smartphone and show them an online article such as http://www.ucmp.berkeley.edu/education/events/deamer1.html which provides free access to a section of the the aforementioned publication where Professor Deamer makes his opinions clear.
At this point you could ask the Witness for their contact details and arrange to revisit them once they have had a chance to read the references and find out if they think Professor Deamer's remarks really should be used to support an implication that the complexity of even single celled organisms can be used to conclude that they could not have emerged from processes of abiogenesis hypothesised by expert researchers in the field.
