Q: thing or things? 
My friend, Hugh, has always been fat, but things got so bad recently...

This is found in my textbook.
Why does the writer use the plural form of thing? He's been fat is just one single thing, isn't it?

A: Things are pretty good now, things are pretty bad now, how are things?, things have been pretty difficult for me lately etc. are colloquial English expressions. I can't tell you exactly why, but it should always be things and those "things" don't really refer to anything concrete or specific. Things just means the current situation or the current state of affairs in a broader sense. That last statement in your example can be paraphrased like this: life has gotten pretty bad for him recently. We're talking about life and its different aspects, whatever those aspects might be, in general. I think, because there are usually many different aspects to life, that's probably why things is plural.
Dialogue example:

— Hey, John. How's it going? I heard you were sick.
  — Yeah, but things are better now. I'm recovering.

No, we're not talking about "things" that the guy in this dialogue supposedly has. What we're saying here is that life is getting better for him and his health is improving because he's on the road to recovery. Things here simply refers to the different unspecified aspects of the current situation (whatever those aspects might be: life, health etc.), but not to anything concrete or specific. So, don't try to overthink it.

A: Check out the definition in the Cambridge Dictionary- in particular the section headed SITUATION. It states that, when we use this word to talk about a situation, we always use the plural things. For example:

We had a few drinks, and then things got out of control.

In your sentence, the person is talking about his friend's weight problem as a situation, so he or she uses the plural things.
The usage of the plural is probably because a situation is generally made up of a number of separate factors, and these factors are the things that we refer to. 

A: You can exchange things with events:

My friend, Hugh, has always been fat, but things got so bad recently...
My friend, Hugh, has always been fat, but events got so bad recently...

More than one thing or event has happened recently. You can change the sentence so that only a single thing or event has happened:

My friend, Hugh, has always been fat, but a thing happened recently...
My friend, Hugh, has always been fat, but an event happened recently...

This changes the meaning from the first pair of sentences and illustrates that thing and event are interchangeable in these sentences. The thing or event is not that Hugh is fat, it's what happened while he was fat. This is why we use the plural, not the singular.

A: If the point is specifically about Hugh's weight, ike he's just put on another 20 lbs, then it might be clearer to say "...it got so bad recently".
"Things" is more about the broader, more abstract situation. Which certainly could be appropriate if Hugh developed health problems or other problems related to his weight. 
