This is one of those... "If 'ifs', 'ands' or 'buts' were cherries and nuts, we'd all have a Merry Christmas"... type of question
Too many variables to take into consideration even to get a "guess"-timate. Time of day, time since food last eaten, what food was last eaten, what exercise, level of insulin sensitivity, sugar fix ratio (i.e. how many grams of carbs raises one's BGL by how much), medical history and drug contraindications, Type 1 or Type 2, on insulin.... and more.
I think it's safe to assume that her BGL wasn't over 141 when she felt low but, how low she actually was? Only a test meter is accurate enough to determine. A thorough analysis of her BGL log might give an idea of how much a fixed amount of sugar usually raises her BGL and a guess-timate can be calculated from that but, you are correct... when she feels low is when she should test. How else to know how much sugar to take in to bring BGLs back up to normal (if hypo)?
For me, I HAVE TO test before fixing a low (rare occasions excepted). If I didn't, I might as well throw a dart at a board and let it tell me what my BGL is at and what to eat. Tried it - it's a recipe for disaster.