Iamindeedawidowwoman - It is very possible that the principal facts mentioned here were real, and that Joab found out a person whose circumstances bore a near resemblance to that which he wished to represent.
Matthew Henry
Concise Bible Commentary
We may notice here, how this widow pleads God's mercy, and his clemency toward poor guilty sinners. The state of sinners is a state of banishment from God. God pardons none to the dishonour of his law and justice, nor any who are impenitent; nor to the encouragement of crimes, or the hurt of others.
SDA Bible Commentary (limited)
Volume 1-7
5. A widow woman. The case was sufficiently unlike that of David so that her story created no suspicion. There were certain basic features in the narrative that were to settle the issue in the mind of David, and on those the emphasis would be placed.