Taking anything is too high of a dose can be dangerous. B6 can have potential nerve toxicity when taken in high enough doses (500mg/day and over although some recommendations suggest toxicity levels are as high as 2000mg/day and over). Generally speaking, it is common to see a B-complex supplement that can contain up to 100mg of B6 per dose. One important thing to consider is the form of B6. Pyridoxine, the basic form of B6 is the one that is known to be toxic in mega doses. However, many high quality supplements contain pyridoxine-5-phosphate (P5P) which is the activated form of B6. This type of B6 is not known to have any toxicity in mega doses.
B12 is another B-vitamin often giving in very high doses. There has been little reported about B12 toxicity and it is very common to see doses almost 1000x the recommended RDA! The amazing thing is the body can tolerate this very well and doses that high can often be effectively used to help improve certain conditions.
All the B-vitamins are water soluble which make them very easy for the body to process (like Vitamin C). From the current information we have, someone would have to be taking very high amounts of B-complex (higher than what is available in most supplements) in order to reach a level of toxicity.
Your wife should look at all her supplements and add up the amount of each B vitamin. A very conservative approach would be to keep levels of Thiamin, Riboflavin, B6, Niacin and Pantothenic Acid under 100mg (milligrams) and levels of B12, and Biotin under or at 1000 mcg (micrograms).
Most likely she is consuming safe levels because B-vitamins are generally thought to be one of the best tolerated family of water soluble nutrients.