Am I right in thinking this is a marine tank? If so, topping up with tap water will lower the SG or salt content of the water. This is alright, if topping up after water has evaporated, but RO would be a better option (specialy purifyed water). However, if the tank/filter leaked, and you replenished the lost water with tap water, the SG will have been lowered. If the trigger has perked up again, it is not too likely to be too low, but it may be a problem that your friend needs to sort out when they get home. Let them know about the problem when they get back, so thet they can rectify any lack of salt issues before the fish come to harm. Moving the SG up again, if you are not an experienced fishkeeper, could do more harm than good, as it is a delicate operation that has to be done slowly. Raise the SG too fast, and you could loose fish. Do you know any marine fish keepers in the area. I you do, you could enlist their help, if not, leave the SG for you frend to sort out if they are comming home soon.