I made some homemade ginger beer using a ginger bug and bottled in a glass flip top bottle. I brought a bottle to a friend we drank half and it has now been in the fridge sealed for a month and a half. I know that refrigeration slows down but doesn’t stop fermentation. Do i need to be concerned about it exploding if/when i try to open it now? Are there safe ways to slowly release the gas? I’ve made this recipe a bunch of times and never had a problem, just feeling unsure about the length of time sealed in the fridge.
If the bottle is only half full then it would have to produce a lot of gas to reach a dangerous pressure since there is more volume to fill. Proper refrigeration should stop fermentation virtually completely and opening the bottle releases the pressure - it might gush everywhere but exploding is very unlikely. You might cover the bottle with a damp tea-towel/cloth in the fridge before picking it up if you are really nervous but it seems very very unlikely to me.
There are two ways a bottle can explode, fast and slow.
Fast happens when there is still too much sugar in the solution and carbonation is still going on. This will mostly happen fast, surely within a week or two. The pressure must become really big for that, though. This would be an amount of sugar greater than 9g/l (to be sure, some bottles can withstand more).
Slow happens when the pressure is correct, but the bottle might contain a mechanical failure. I had this with a saison beer. One bottle was neatly separated from its bottom.
Bottom line: I don't think you have anything to fear. Opening the bottle will not make it explode, on the contrary. Opening the bottle will always immediately release the pressure. And if you want to make sure, wrap your bottle in a towel when handling it for opening.
Update: since you confirmed that the bottles are beer flip-top bottles, you can be sure that they can withstand a whole lot of pressure.
Simple answer is that you can't know when or if they'd explode.
If you had taken a gravity reading before bottling you could have calculated how much sugar was left and deferred to the many guides that help you determine how much sugar to add for bottle carbonation
When the bottles are in the fridge you are right that they never truly stop fermenting, but the yeasts are pretty close to dormant! So even a month is not going to change a whole lot.
Since they are still fine now you could simply pasteurize them, that way you've contained the danger of future bombs. That would kill the beneficial live things in the ginger beer though.
I've had a few times when I did not drink my ginger beer quickly and got slightly nervous. In those cases I just slightly move the metal 'swing' while holding down the top with my hand. This way I can in a controlled manner let out a little bit of the pressure without risking to have a flat ginger beer later on.