A sensor has detected an unsupported liquid in that tank. It could be legitimate or possibly a sensor failure, such as maybe the NOx sensor (which is external of the tank in the SCR system) or another sensor within the DEF tank.

A barrage of questions that come to mind.

What model year?
What or where did he last fill DEF?
Was it raining during that fill?
How many miles since the last fill?
Was this DEF in the jug from Ford or a parts store or from a truck lane DEF pump?
When you command a system check, what level is the DEF? (full, under half, X MTE)

 

This will not be a covered warranty repair.

Probably just need to drain and clean the tank.

It should be scanned for codes as a first step along with the very good questions above.

 

 

That lower fuel bowl (DFCM) is fuel only, which sends fuel to the secondary filter in the engine bay. As an important side note, if DEF is in that, things aren't good and you don't want to drive it or turn on the key at all. That's a flat bed tow only situation.
As long as the message you've received is the only warning, then your issue is limited to the DEF system. That message isn't because of fuel contamination (in the fuel system).

2013 is past the firmware update I was thinking existed for the 2011's.
Same brand of DEF he has always purchased?
I've not seen DEF in a chain parts store that isn't supported for well known SCR systems but I guess I can't say no such product exists either.

See if the parts store can read codes for you if you don't have access to a bluetooth OBDII reader/phone app. If the "reductant" system threw a code (DTC), one of us can at least provide the generic description of that code.
Depending on the code, I think you need to follow what Senix mentioned and drop/clean the tank and refill with known approved DEF. Or get it to a diesel shop or dealer who knows what to do with the DEF/SCR system on these HD class trucks.
The reason for getting the code first, what if this error is simply the result of another issue with that system?

 

To be funny, you could ask your husband if he peed in the DEF tank. Stranger things have happened. Recently, there was a guy on the forum that made his own DEF at home. It didn't work out so well.

 

Get one of the small hand pumps from Wal Mart or Harbor Freight. Pump the DEF out of the tank and hten refill with fresh DEF. Clear DTC's and see if this cures the problem. Otherwise might be more involve and fairly expensive.