That friend of yours is either lying to you. Or is pushing a lie onto you without bothering to check it. Then you passed it on here without bothering to check it either.
And it's not hard to check.
The reality is that Honda are restructuring and the EU's new Free Trade Agreement with Japan means they are re-evaluating their global manufacturing base. Cars built in Japan will have tariff-free access to the EU market in 2021; cars built in the UK will not, assuming brexit happens.
This is clearly one factor in the decision, even though Honda are too politically sensitive to make that statement.
There are other factors aside from Brexit no doubt. But "diesel" is not one of them.
The Financial Times contacted them and put that directly to them. The article itself is paywalled, but the FT's chief political correspondent tweeted the headline figures:
“For financial year 2018/19, the breakdown of cars built at Honda of the UK Manufacturing Ltd is Diesel 6% and Petrol 94%.”
https://twitter.com/PickardJE/status/1097629287782514688
“For financial year 2018/19, the breakdown of engines built at Honda of the UK Manufacturing Ltd is Diesel 12% and Petrol 88%.”
https://twitter.com/PickardJE/status/1097630309120647170
So to clarify. You posted that it was a "diesel only plant" based on hearsay. In fact, only 6% of cars produced there last year were diesel.
Again -- Honda are NOT blaming Brexit on the plant closure. They talk about "global restructuring" and "reassessing market access". But if the last of those is not a code for "brexit" then I don't know what it is... and I'm very sceptical that a plant that Honda invested hugely in, just 3 years ago, would be getting closed as part of the "global restructure" if it was NOT for brexit.
----------
(edited for spelling right after posting, and adding tweet URLs for source)