A warm welcome Faithful Witness.
What a wonderful calling and so too your embracing it.
I like Vanderhoven7's suggestion of getting them to use scripture to validate their assumptions.
Many have observed that simple questions work best.
The simpler the better.
The fewer the better.
This helps them focus - something the WBTS tries to avoid at all costs.
Asking them to explain "law vs grace" from their heart, might for example be broken down into stages over time.
They first need time to reflect on why they have no idea what "grace" is.
So one might ask them to explain "grace" from their heart.
They wouldn't be able to.
You might gently and patiently introduce them to their own "Watchtower Library Online" and how to use its search facility.
You might get them to search for these key words: grace boast gift
WOL (library) search link for "grace": http://wol.jw.org/en/wol/s/r1/lp-e?q=grace+boast+gift&p=doc
You might even go much simpler.
For example inviting them to "help" you figure out if it is true that more than half their Bible's references to "Good News" are by Paul.
If your JW "friends" are at least able to read, and count up to 152, this could be a real confidence and morale booster for them as they get to "help" you answer your "difficult" question.
Of course this begs the question why the "Good News" according to Paul is not in their hearts or on their lips.
WOL (library) search link for "Good News" in the NWT alone: http://wol.jw.org/en/wol/s/r1/lp-e?q=%22good+news%22&fc[]=bi
May your patient efforts be rewarded, and encourage others to demonstrate the same care, concern and compassion for our dear lost ones.
Greetings and blessings
Fernando
South East QLD, Australia
(Left the WBTS for Jesus and the "gospel of grace" mid-2009. Now identify as SBNR.)