Diesel fuel is used as part of the dpf regeneration cycle.Looks like something has gone wrong with yours that has caused diesel to fill the cylinders to excess and then find its way into the oil sump.

The diesel gets into the oil when the diesel particle filter (DPF) is regenerating, i.e. burning off the soot that it traps in the exhaust. The ECU causes extra diesel to be injected during this phase to start the burn-off happening in the DPF canister. If the DPF isn't regenerating properly, then the ECU will keep trying and injecting diesel, and this surplus diesel eventually finds its way into the lubricating oil past the piston rings.

The Mondeo does have an engine oil quality sensor which SHOULD indicate when the oil has been diluted. I guess the message in the dash was trying to tell you this.

I owned a 2008 2.0TDCI Mondeo from new, and after 3,000 miles from new, I had the "engine oil service required" message come up in the dash. The supplying dealer told me they had a Ford service bulletin about this, that said the oil quality sensor was over-sensitive and needed an ECU update. They did this and changed the oil (presumably as a safeguard). I never saw that message or had a DPF problem for the next 36000 miles and 3 years.

Perhaps the dealer assumed this update had been done, but I am surprised they never checked the oil at all. Did you ever check the oil level?