My dad (72 yrs old) has rectal cancer. He's on
alternative medicine(Cellect-Budwig protocol) for the past few months.

His haemoglobin was low (7.3) as per his previous testing so he was on various foods to improve his iron and haemoglobin level in his body. One of these was green
juices (spinach,mint and cilantro).

After a few days (his haemoglobin level went upto 8.3) he started having symptoms like shivering of hands, muscle pain (unable to stand or walk on his own without
support) and some memory loss.

We had his vit B12 levels tested and they came out higher than the acceptable limit.

Could the above symptoms be due to the cilantro pulling out heavy metals and redistributing them in the body? (he did use a considerable amount of cilantro each time).

You should not be guessing on this.  These can also be stroke symptoms.

I don't know if he was on blood thinners at all, but if so cilantro would have definitely been contradicted since it contains blood thinning coumarins.  And the most common blood thinner used is Coumadin (Warfarin), which can cause stokes on its own.  Adding other blood thinners will only increase the risk.  Even if he is not on blood thinners there are other causes of strokes and they should have checked him for this immediately based on those symptoms.  If it was stroke then the longer he goes without being addressed the worse and more permanent the symptoms may become since most of the damage comes from enzymes released after the stroke.