have these avocados gone bad?

Your friend is right. The avocado is not spoiled or in any way dangerous, just changed during storage. You can eat it without any ill effects.

There are people who would throw food out for cosmetic reasons, you can do that too, if you prefer.

Avocados are frequently bad in one or both of two ways:

• "Bruising" where the flesh turns dark brown/black, is squishy and has a sour smell;
• "Fibrousness" where the flesh gets full of green or brown fibers.

In the bruising case, you can eat the rest of the flesh if you can cut out the bruises, at least until it takes over the avocado, and then you compost it. Bruising is caused by basic rot.

In your case, though, you're seeing the fruit's fibers from when it formed, because of being picked too young, or coming from a stressed tree, or just as a mutation. Per California Avocados:

Strings or stringy fruit or the thickening of the vascular bundles (fibers that run longitudinally through the fruit) are generally the result of fruit from younger trees or improper storage conditions. Often times the fibers or strings will disappear or become less noticeable as the fruit (and tree) matures.

These strings are harmless, but they can impair the taste and texture of the avocado, so I'd recommend giving it a taste to decide if you can still use it. It also depends partly on what you're using it in, and how you plan to process it.

That looks like a Haas avocado. You can tell they are ripe by giving the wider end a gentle squeeze. If it yields to a firm, gentle squeeze, it is good to go. If it is very soft, it has probably ripened too much. When you cut it open, a ripe avocado is uniformly green. There is generally no space between the skin and the flesh. Once they over-ripen they get brown spots inside. The taste also goes quite bitter.