Here are the texts, and there's a satisfying update on the story at the bottom.
UPDATE: “Many recommended a small claims court, which would help in small legal dealings such as this one. After speaking with my sister about her options we decided that this is what we would do as a last-case resort. Yesterday, we decided to call the husband on his personal phone. It took a few tries to get through to him, but he eventually picked up and was quite sheepish about the whole thing. Basically, the conversation said something to the effect of ‘yeah, I’ll talk to her about it.'”
“Since […] he was ignoring [us], we decided to send him the following text:
‘Mr. [Name], we are aware you are reading our messages. We have consistently tried to reason and work with you, and it is honestly quite laughable that you are refusing to pay. After consultation with friends^^these other friends were reddit users!^^we have decided we will be using a Small Claims Court if you do not return us the $128 in cash that you rightfully owe. You will have to go through legal fees and court proceedings and it will certainly cost you more than what you owe us. So, as soon as you and your wife stop acting like children and face the people you owe money to in person, we may be able to have a reasonable discussion.’
Guess what happened? Within thirty minutes, my sister was unblocked from the mother’s phone and the cash is set to return today at 2 pm.”