Two problems with this
do you think it's wise idea to call her while you are attempting to ctb because whichever you look at it, it'll traumatize her should you get help after taking SN or if you actually do ctb and pass on. I just personally wouldn't risk putting someone through that, be it an ex especially since she'll possibly blame herself for not noticing something off while you are talking to her about the afterlife and such.
Drinking it and then going for a walk is a terrible idea, you may not even be able walk a few steps before you are out (this has been noted in a few attempts where people recounted going to the bathroom and they made a noise by passing out and waking up and then going unconscious again in the ambulance and then waking up in ER but still delirious before being administered M-Blue so they were out before they even knew it). It is recommended to stay put in one place, be it a bed or something and make sure nothing else is around you so you don't hit it by accident and cause attention to be drawn to you under any circumstance unless you think you should call for help and the attempt has to be aborted.
You won't just pass out either, although it could happen depending on whether or not you even vomit sn and if it could get a bit uncomfortable for you, thing is the onset of symptoms before unconsciousness is rather quick. Getting dizzy and feeling heavy and even hot or sweaty is common although it isn't a painful thing at all, the only you may consider uncomfortable is your heart racing quite a bit although sometimes I've seen in medical articles that when some people were found, their heart rates were not too high and they were unresponsive for the most part, anyway, the only way you'll even get close to just "passing out and that's it" is if you get Benzos to take before SN.
There's a reason why an SN attempt should be as quietly done as possible, as to not alert anyone to what you are doing because you may not even get a chance to survive considering the fact that high amounts of SN taken is rather difficult to reverse which is why, contrary to people questioning its reliability, it is a very lethal method.