Nope. If the risk of a child being r@ped can be almost wholly eradicated by avoiding strange men when they are 3 years old, then of course it can, must and should be.
In safeguarding, risk decisions consider both likelihood and severity of harm. Low impact risks with minor consequences may be reasonably tolerated, eg gran is likely to feed the child sugar so it is ok to leave the child with gran even though that is not a desirable oucome.
But where the potential outcome involves serious or irreversible harm to a child, such as being r@ped in a tolet, even a low statistical probability wholly justifies the precautionary approach.
For intimate tasks such as toileting or bathing, it is therefore appropriate to limit care to trusted, and KNOWN and appropriately authorised adults whenever possible.
The mother gets to decide who is a trusted known and appropriately authorised adult when the possible risk to a 3 year old, however tiny, is to be r@ped, sexually assaulted or otherwise abused in a toilet.
Gran is wrong, gran is arguing despite being wrong, and as she made a pointlessly stupid and easily avoidable decision gran no longer gets unsupervised authority over the child. That's the only logical and safe outcome.