Also, people don't really purposefully buy fentanyl. People buy things they think are something else ... that then turn out to be tainted with fentanyl (typically heroin) or made of fentanyl (typically 'pirated' hydro/oxy pills whose active ingredient is actually fentanyl ... Prince died from those).
$40 every few days will support a relatively small scale opioid habit. A serious habit is going to run you more like $80-$150/day. And that will be like EVERY day, without fail.
Someone taking out cash very regularly is definitely a red flag unless you know of some other logical explanation.
Here's one thing to try ... see what the pattern is ... if it's every 48 hours or so, try to reach her after like 42 hours. See if you can. See what her mood is when you do. Does she try to get you off the phone unusually quick or not answer? That's a possible sign she's focused on getting her next packet and possibly in withdrawals.
If an EXTRA day suddenly goes by withOUT her taking out money, definitely try calling her then. If she doesn't answer at all ... maybe she's sick and not caring about anything but getting more.
In my using days, if I was out and starting to get sick? I didn't want to talk to ANYBODY ... who was not playing a productive role in the process of getting more dope. They were NOBODY at that moment.
Also try calling her like 3 hours after she takes out money. How does she sound/act then?